Proton-Induced Displacement Damages in 2-D and Stacked CMOS SPADs: Study of Dark Count Rate Degradation

Dark count rate (DCR) degradation is measured on 40-nm 2-D and stacked CMOS single-photon avalanche diodes (SPADs) after proton irradiations. Mean DCR increase is plotted for different displacement damage doses (DDDs) for two biasing conditions. Due to field enhancement effects such as Poole–Frenkel and phonon-assisted-tunneling (PAT), non-ionizing energy loss (NIEL) scaling is found to be dependent on the excess bias. Moreover, the associated damage factors are higher compared to pixels with unity gain photodiodes. A model predicting DCR increase with DDD based on field enhancement factor equations and damage energy deposited by the proton-induced recoil spectrum is established and fits well with experimental data. Finally, activation energies are extracted: low values between
$25 ^{\circ} \text{C}$
and
$60 ^{\circ} \text{C}$
confirm the domination of field effects over thermal generation. Furthermore, large energy discrepancies seen at a fixed DCR increase are suggesting that defect structures could also have an impact on DCR distributions.